Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain and give mechanism.
Addition of HBr to propene in the absence of peroxide follows the Markovnikov rule, where the negative part of the addendum attaches to the carbon with fewer hydrogen atoms. This yields 2-bromopropane as the major product.
The mechanism involves electrophilic addition. HBr provides an electrophile, , which attacks the double bond to form carbocations. The secondary carbocation is more stable than the primary carbocation and forms faster. It is then attacked by to form 2-bromopropane.
In the presence of benzoyl peroxide, the reaction follows Anti Markovnikov addition, also called the peroxide effect or Kharash effect. Contrary to the Markovnikov rule, this yields 1-bromopropane as the major product.
This peroxide effect proceeds via a free radical chain mechanism. Benzoyl peroxide undergoes homolysis to form free radicals. A bromine radical adds to the double bond, forming a secondary free radical which is more stable than the primary one. This radical reacts with HBr to form 1-bromopropane. This effect is only seen with HBr, not HCl or HI.
Explanation
The answer explains both the Markovnikov and Anti-Markovnikov addition of HBr to propene. Without peroxide, the reaction proceeds via a carbocation intermediate where the more stable secondary carbocation leads to 2-bromopropane. With peroxide, it proceeds via a free radical mechanism where the more stable secondary free radical leads to 1-bromopropane (Kharash effect).